Feedback active noise reduction headphones include supra-aural headphones and circum-aural headphones. The open-loop transfer function of supra-aural headphones has poor stability. In designing the feedback noise reduction, the stability in various situations should be considered, and the noise reduction amount of the headphones has to be sacrificed to ensure stability. In addition, a significant characteristic of supra-aural headphones that distinguishes them from circum-aural headphones is their small volume. Installing a noise reduction microphone directly in front of a speaker will increase the thickness of supra-aural earphones or result in wearing discomfort. In conclusion, supra-aural feedback active noise reduction headphones have not been extensively used and popularized.
Circum-aural feedback active noise reduction headphones generally have a relatively large volume, so sealing is an important factor to be considered in designing. As the earmuff is airtight, a relatively rigid cavity will be formed after wearing, within which the intensive sound wave reflection will cause the howling of the feedback active noise reduction headphone. In order to absorb and decrease the sound wave reflection within the cavity, relatively thick felted wool or compressed sponge is usually used to fill the interior. The filler is distributed between a speaker and an ear canal opening of the wearer, and serves to protect the speaker and the noise reduction microphone and reduce the internal reflection of the walls, but at the same time a noise reduction amount at the ear canal opening of the wearer is considerably reduced.
It is desirable to provide a solution that addresses the above technical problems. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.